Semiconductor device and manufacturing method thereof

ABSTRACT

The invention provides a novel memory for which process technology is relatively simple and which can store multivalued information by a small number of elements. A part of a shape of the first electrode in the first storage element is made different from a shape of the first electrode in the second storage element, and thereby voltage values which change electric resistance between the first electrode and the second electrode are varied, so that one memory cell stores multivalued information over one bit. By partially processing the first electrode, storage capacity per unit area can be increased.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device which storesmultivalued data. The present invention also relates to a semiconductordevice including a circuit formed of a storage element and a thin filmtransistor (hereinafter referred to as TFT) and a manufacturing methodthereof.

Note that in this specification, a semiconductor device refers to adevice in general which functions by utilizing semiconductorcharacteristics. An electro-optic device, a semiconductor circuit, andan electronic apparatus are all semiconductor devices.

2. Description of the Related Art

In general, a storage device (also referred to as a memory device)includes a memory portion which stores data and a peripheral circuit (adriver, a decoder, a sense amplifier, or the like) for reading/writingdata from/to the memory portion. In a conventional storage device, anarea required to store one bit is larger than the size of one switchingelement (typically, a field effect transistor). Therefore, an arearequired to store one bit depends on processing technique formanufacturing a transistor, which prevents realization of alarge-capacity storage device.

In recent years, as application software gets complex or the like, largecapacity and high integration are highly demanded for a memory.

Patent Document 1 discloses a cell structure of a memory which isprovided with an impedance phase change film formed of an organicmaterial between electrodes. The memory has a structure where the filmthickness of the organic material or contact areas with an electrode arevaried in one memory cell, and a writing voltage is set at a pluralityof impedance state transition points in hysteresis characteristics, sothat information which can be stored in one memory cell can be mademulti-valued.

-   [Patent Document 1]

Japanese Published Patent Application No. 2001-189431

SUMMARY OF THE INVENTION

The invention provides a novel memory capable of storing multivaluedinformation with a small number of elements, which uses a relativelysimple process technique.

One of the objects of the invention is to provide a storage devicehaving a high integration degree per bit, that is, a storage device withlow cost per bit. Another object of the invention is to provide astorage device with low power consumption by reducing the number ofcircuit elements and the number of wirings per bit.

In view of the foregoing problems, the invention provides a memorydevice in which a plurality of regions broken down (or changed) by adifferent voltage are formed in one memory cell and the memory cell ismultivalued in the case where a memory element provided with a materiallayer between a pair of electrodes is formed and an operating methodthereof.

Note that breakdown of a material layer of a memory element means thatconductive layers (electrodes) which interpose the material layer of thememory element are short-circuited. As breakdown of a material layer ofa memory element, dielectric breakdown is taken as an example. Further,a state of a material layer of a memory element is changed by heating toa glass transition temperature or higher to be softened or melted, andas a result, conductive layers which interpose the material layer of thememory element are short-circuited in some cases.

Note that a change of a material layer of a memory element means thatelectric characteristics of a material layer of a memory element ischanged by applying a voltage. As an example, a phase change memoryelement which includes a material layer having electric characteristicsto be reversibly changed by applying a voltage is given.

In the invention, a lower electrode is provided with a step, so that acorner (an edge) is formed; therefore, a voltage value which changescharacteristics of a memory cell can be reduced by electric fieldconcentration at a corner, thinning of an organic layer near the corner,or the like. In addition, the height of the step of the lower electrodeor a cross sectional shape of the lower electrode is changed, so that avoltage value which changes characteristics of the memory cell can bechanged by each region such as a region provided with a step or a regionwith a different cross sectional shape.

By utilizing characteristics described above, a plurality of regionseach having a different voltage value which changes characteristics of amemory cell can be formed in the memory cell. That is, multivaluingoperation (storing multivalued information) in which one memory cellstores more than one bit can be performed.

For example, a material layer of a memory element is divided into threeregions: a first region, a second region, and a third region. Anelectrode in contact with the first region of the material layer of thememory element is provided with a first step. The electrode in contactwith the second region is provided with a second step. The electrode incontact with the third region is not provided with a step. That is, astructure is that a first memory element is included in the firstregion, a second memory element is included in the second region, and athird memory element is included in the third region. The first step islarger than the second step. As the step gets higher, the material layerof the memory element, which is formed over the step, is broken down bya lower voltage value. The first region, the second region, and thethird region are in order of increasing a breakdown voltage value of thematerial layer in the memory element in each region.

The invention is not limited to a memory structure in which an electrodeis provided with a step, and various structures can be employed as longas a plurality of regions each having a different voltage value whichchanges characteristics of a memory cell can be formed. For example,when an electrode is provided with a step, there is a method ofutilizing a difference between taper angles of steps other than a methodof utilizing a difference of the height of steps. A breakdown voltagecan be lowered when a step has a large taper angle, and a breakdownvoltage can be increased when a step has a small taper angle. Stepshaving a different taper angle at a side surface of an electrode areformed in a memory cell, so that the memory cell can be multivalued aswell. In addition, a difference between a step having an approximatelyperpendicular side surface and a step with a small taper angle can beutilized. Note that in this specification, a tapered shape refers to anangle equal to or larger than 5° and smaller than 85° with respect to ahorizontal plane. A step having an approximately perpendicular sidesurface refers to that having an angle equal to or larger than 85° andequal to or smaller than 95° with respect to a horizontal plane.

In addition, a plurality of regions each having a different voltagevalue which changes characteristics of a memory cell may be formed bycombining the structure in which an electrode is provided with a stepand a structure with different taper angles.

In addition, a memory cell in the invention refers to one unit includinga plurality of memory elements, a wiring (or a TFT), or the like. Aplurality of memory cells are regularly arranged to form a memoryportion of a semiconductor device.

In a structure 1 of the invention that is disclosed in thisspecification, a semiconductor device includes one memory cell whichincludes a first storage element and a second storage element. The firststorage element and the second storage element include a common firstelectrode, a common second electrode, and a common material layerbetween the first electrode and the second electrode. In thesemiconductor device, at least a part of a shape of the first electrodein the first storage element is different from a shape of the firstelectrode in the second storage element, and thereby voltage valueswhich change electric resistance between the first electrode and thesecond electrode are varied, so that one memory cell stores multivaluedinformation over one bit. By partially processing the first electrode,storage capacity per unit area can be increased.

In a conventional structure where the film thickness of an organicmaterial varies in one memory cell, it is difficult to adjust the filmthickness of the organic material accurately; therefore, it is difficultto reduce variation of a writing voltage in a plurality of memory cells.On the other hand, in the invention, only a part of the first electrodeis required to be processed; therefore, variation of a writing voltagein a plurality of memory cells can be reduced as etching accuracy ishigher as compared to the conventional structure.

Further, in a conventional structure where an electrode contact area ischanged in one memory cell, the area is greatly increased; therefore, itis difficult to increase storage capacity per unit area. On the otherhand, in the invention, increase of the area can be reduced as comparedto the conventional structure; therefore, increase of storage capacityper unit area can be realized.

In the invention, a first electrode is electrically connected to a wordline and a second electrode is electrically connected to a bit line, sothat a passive matrix type storage portion can be formed. Further, afirst electrode is connected to a switching element, so that an activematrix type storage portion can be formed. In a structure 2 of theinvention, a semiconductor device includes a first storage element whichincludes a first electrode on an insulating surface, a material layerover the first electrode, and a second electrode over the materiallayer; and a second storage element adjacent to the first storageelement. The first storage element and the second storage element havedifferent voltage values which change electric resistance. The secondelectrode of the first storage element is common to the second storageelement. The first storage element and the second storage element areelectrically connected to the same thin film transistor. By electricallyconnecting a plurality of storage elements to the same thin filmtransistor, a driver circuit can be made small and reduction in size ofthe semiconductor device can be realized as compared to a semiconductordevice including a passive matrix type storage portion.

A partition wall may be provided between a plurality of storage elementsin one memory cell. In a structure 3 of the invention, a semiconductordevice includes a first electrode on an insulating surface, a partitionwall over the first electrode, a material layer over the first electrodeand the partition wall, and a second electrode over the material layer.A portion of the partition wall is provided between a first regionsurrounded by a portion of the partition wall over the first electrodeand a second region surrounded by a portion of the partition wall overan end portion of the first electrode. In the first region, at least thefirst electrode, the material layer, and the second electrode areoverlapped with one another. In the second region, at least the materiallayer and the second electrode are overlapped with each other. Byproviding such a partition wall, even if a space between memory cells isnarrowed, a defect such as a short circuit between adjacent memory cellscan be prevented; therefore, high integration can be achieved andstorage capacity per unit area can be increased.

A first electrode may have a stacked-layer structure of two or morelayers in order that a part of the first electrode is easily processed.In a structure 4 of the invention, a semiconductor device includes afirst electrode on an insulating surface, a partition wall over thefirst electrode, a material layer over the first electrode and thepartition wall, and a second electrode over the material layer. Thefirst electrode has a stacked-layer structure of two or more layers. Inone memory cell, a first region surrounded by a portion of the partitionwall over the first electrode, a second region where an end portion of abottom layer of the first electrode and the material layer areoverlapped with each other, and a third region where the material layerand an end portion of a top layer in the stacked layers of the firstelectrode are overlapped with each other are included. Portions of thepartition wall are provided between the first region and the secondregion; the second region and the third region; and the first region andthe third region; respectively. In the first region, at least the firstelectrode, the material layer, and the second electrode are overlappedwith one another. In the second region, at least the material layer andthe second electrode are overlapped with each other. The end portion ofthe bottom layer of the first electrode and the end portion of the toplayer thereof are in different positions. By using the stacked-layerstructure of two or more layers for the first electrode, even if asurface of the first electrode has a complex shape, the first electrodecan be formed accurately by adjusting an etching condition and amaterial of the stacked layers, and variation of a writing voltage of aplurality of memory cells can be reduced.

Further, in the structure 3 or 4, the semiconductor device may alsoinclude a thin film transistor on the insulating surface, and the firstelectrode may be electrically connected to the thin film transistor toform an active matrix type storage portion. In addition, in thestructure 3 or 4, the semiconductor device may include a thin filmtransistor and an antenna on the insulating surface, and the firstelectrode may be electrically connected to the thin film transistor anda circuit including the thin film transistor may be electricallyconnected to the antenna so that communication with a radio signal canbe performed. As a circuit which is electrically connected to theantenna, a writing circuit, a reading circuit, a sense amplifier, anoutput circuit, a buffer, or the like is taken as an example.

Further, in each structure described above, the first electrode mayinclude portions each having different film thickness and at least onestep. Alternatively, the first electrode may include portions eachhaving different film thickness and at least two side surfaces havingdifferent taper angles.

Further, in each structure described above, one memory cell may includea plurality of regions over the first electrode, and one memory cell canmemorize a plurality of bits.

Further, in each structure described above, the material layer mayinclude an organic compound. Thus, if another person disassembles thememory cell so as to commit forgery, forgery can be extremely difficultsince an organic material is easily altered on exposure to air or thelike so that the material is not easily identified.

As a material for the material layer of the memory element in theinvention, a low molecular material, a high molecular material, asinglet material, a triplet material, or the like may be used. As thematerial for the material layer, a substance having a high holetransporting property can be used, for example, an aromatic amine-basescompound (namely, a compound having a bond of a benzene ring andnitrogen) such as 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl(abbreviation: α-NPD),4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl (abbreviation:TPD), 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine (abbreviation:TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine(abbreviation: MTDATA), or4,4′-bis(N-(4-(N,N-di-m-tolylamino)phenyl)-N-phenylamino)biphenyl(abbreviation: DNTPD); a phthalocyanine compound such as phthalocyanine(abbreviation: H₂Pc), copper phthalocyanine (abbreviation: CuPc), orvanadyl phthalocyanine (abbreviation: VOPc), or the like can be used.Further, an organic compound material having a high electrontransporting property can also be used, for example, a material formedof a metal complex or the like having a quinoline skeleton or abenzoquinoline skeleton such as tris(8-quinolinolato)aluminum(abbreviation: Alq₃), tris(4-methyl-8-quinolinolato)aluminum(abbreviation: Almq₃), bis(10-hydroxybenzo[h]-quinolinato)beryllium(abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation:BAlq), a material formed of a metal complex or the like having anoxazole-based or thiazole-based ligand such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂), orthe like can be used. Other than the metal complexes, a compound or thelike such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(abbreviation: PBD),1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), or bathocuproin (abbreviation: BCP) can beused. In addition, as the material layer, a material partially includingan inorganic compound may be used as well as a material formed of onlyan organic compound material.

In order to prevent manipulation or unauthorized use of information,when an organic material or an inorganic material of which phase doesnot change reversibly is used for the material layer of the memoryelement, writing to a memory can be performed only once.

When an organic material (for example, bathophenanthroline(abbreviation: BPhen)) or an inorganic material (such as tellurium(Tel), tellurium oxide (TeOx), antimony (Sb), selenium (Se), or bismuth(Bi)), of which phase changes reversibly, is used as the material layerof the memory element for repetitive use, rewriting data to the memorycan be performed multiple times. Further, a reader/writer may be able towrite to and read from the memory element including an organic material.

According to the present invention, a memory element can be multivalued.That is, in a memory portion where a plurality of memory elements arearranged, storage capacity per unit area can be increased.

The memory element is multivalued, which enables high integration;therefore, the area of the memory element can be reduced.

In addition, the memory element in the invention can be formed over thesame substrate where a circuit for controlling the memory element isformed through the same step that is a part of all steps; therefore, asemiconductor device including the memory element can be manufactured atlow cost.

Further, the memory element in the invention can be provided over aresin substrate by using a separation method or a transfer method;therefore, a semiconductor device including the memory element can bemade thinner and lighter and impact resistance thereof can be improved.

In addition, by forming the memory element in the invention and anantenna over the same resin substrate, the number of steps can bereduced and a semiconductor device having excellent impact resistancecan be completed.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1D are views showing a manufacturing step of a semiconductordevice of the invention.

FIGS. 2A and 2B are views showing a manufacturing step of asemiconductor device of the invention.

FIG. 3 is a view showing a manufacturing step of a semiconductor deviceof the invention.

FIGS. 4A and 4B are views showing a manufacturing step of asemiconductor device of the invention.

FIGS. 5A and 5B are views showing a manufacturing step of asemiconductor device of Embodiment Mode 2.

FIGS. 6A and 6B are views showing a manufacturing step of asemiconductor device of Embodiment Mode 2.

FIG. 7 is a view showing a manufacturing step of a semiconductor deviceof Embodiment Mode 2.

FIGS. 8A and 8B are views showing a manufacturing step of asemiconductor device of Embodiment Mode 2.

FIGS. 9A and 9B are views showing a manufacturing step of asemiconductor device of Embodiment Mode 3.

FIG. 10 is a view showing a manufacturing step of a semiconductor deviceof Embodiment Mode 3.

FIGS. 11A and 11B are views showing a manufacturing step of asemiconductor device of Embodiment Mode 4.

FIGS. 12A and 12B are views showing a manufacturing step of asemiconductor device of Embodiment Mode 4.

FIGS. 13A and 13C are top views of a semiconductor device of EmbodimentMode 2. FIG. 13B is a cross sectional view of a semiconductor device ofEmbodiment Mode 2.

FIGS. 14A to 14C are views showing a manufacturing step of asemiconductor device of Embodiment Mode 5.

FIGS. 15A and 15B are views showing a manufacturing step of asemiconductor device of Embodiment Mode 5.

FIG. 16A shows a structure example of a semiconductor device of theinvention.

FIG. 16B is a view showing an electronic apparatus including asemiconductor device of the invention.

FIGS. 17A and 17B are views showing an electronic apparatus including asemiconductor device of the invention.

FIGS. 18A to 18F are views showing usage of a semiconductor device ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment modes of the present invention will be hereinafter describedwith reference to drawings. However, the present invention can beembodied in many different modes and it is easily understood by thoseskilled in the art that the mode and detail can be variously changedwithout departing from the scope and spirit of the invention. Therefore,the present invention is not construed as being limited to descriptionof the embodiment modes. Note that the same portions or portions havinga similar function are denoted by the same reference numeral anddescription of such portions is omitted.

Embodiment Mode 1

In this embodiment mode, a method of manufacturing a semiconductordevice including a memory element over a glass substrate as aninsulating substrate is described. A method of forming a material layerof the memory element over an electrode step is described. Note that amode in which the memory element and a circuit (a control circuit) forcontrolling the memory element are formed over the same substrate isshown.

First, as shown in FIG. 1A, a separation layer 402 is formed over aglass substrate 401. Quartz or the like is used for the insulatingsubstrate as well as glass. As the separation layer 402, a filmcontaining metal or a film containing silicon is formed entirely orselectively over the substrate. By at least selectively forming theseparation layer, the glass substrate 401 can be peeled off later. Asthe metal, a single layer or stacked layers of an element selected fromW, Ti, Ta, Mo, Nd, Ni, Co, Zr, Zn, Ru, Rh, Pd, Os, and Ir, or an alloymaterial or a compound material containing such metals as a maincomponent can be used. As the compound material, oxide or nitride ofsuch metal may be used. In addition, the state of a film containingsilicon may be any of a crystalline state, an amorphous state, and amicrocrystalline state. A speed to remove the separation layer 402 canbe controlled in accordance with the state.

Next, an insulating layer 403 is formed so as to cover the separationlayer 402. The insulating layer 403 is formed of silicon oxide, siliconnitride, or the like. Then, a semiconductor layer is formed over theinsulating layer 403, and the semiconductor layer is crystallized bylaser crystallization, thermal crystallization using a metal catalyst,or the like, and then patterned into a desired shape to form anisland-shaped semiconductor layer. The laser crystallization may beperformed by using a continuous wave laser or a pulsed laser. As thelaser, one or more of the following can be used: an Ar laser, a Krlaser, an excimer laser, a YAG laser, a Y₂O₃ laser, a YVO₄ laser, a YLFlaser, a YAlO₃ laser, a glass laser, a ruby laser, an alexandrite laser,a Ti:sapphire laser, a copper vapor laser, and a gold vapor laser. Forexample, a pulsed excimer laser can be used. The semiconductor layer isformed to have a thickness of 0.2 μm or less, typically 40 to 170 nm,and preferably 50 to 150 nm. Note that for the semiconductor layer, anamorphous semiconductor, a microcrystalline semiconductor, an organicsemiconductor, or the like may be used as well as a crystallinesemiconductor. In addition, the semiconductor layer may be formed byusing a material containing silicon, for example, by using a mixedmaterial of silicon and germanium.

Next, a gate insulating layer 405 is formed so as to cover thesemiconductor layer. The gate insulating layer 405 is formed usingsilicon oxide, silicon nitride, or the like. The gate insulating layer405 may be formed by a CVD method, a thermal oxidation method, or thelike. Alternatively, the semiconductor layer and the gate insulatinglayer 405 may be continuously formed by a CVD method, and subsequentlycan be patterned at the same time. In this case, impurity contaminationat the boundary between each layer can be suppressed.

Then, a gate electrode layer 406 is formed. The gate electrode layer 406is formed by using an element selected from tantalum (Ta), tungsten (W),titanium (Ti), molybdenum (Mo), aluminum (Al), and copper (Cu), or analloy material or a compound material including such elements as a maincomponent, and patterned into a desired shape. In the case of patterningby a photolithography method, the width of a gate electrode can be madenarrower by using a resist mask of which width is narrowed by plasmaetching or the like. Accordingly, performance of a transistor can beimproved. The gate electrode layer 406 may have either a single-layerstructure or a stacked-layer structure. FIG. 1A shows the case where thegate electrode layer 406 has a stacked-layer structure.

Next, impurity elements which imparts a conductivity type are added tothe semiconductor layer to form an impurity region 407. The impurityregion 407 is formed by using a resist mask formed by a photolithographymethod and adding an impurity element such as phosphorus, arsenic, orboron. With the impurity element, a polarity of either an n-channel typeor a p-channel type can be determined.

Then, as shown in FIG. 1B, an insulating layer is formed of an insulatorcontaining silicon, for example, silicon nitride, and the insulatinglayer is anisotropically (perpendicularly) etched to form an insulatinglayer (also referred to as a sidewall) 409 which contacts with a sidesurface of the gate electrode. When the sidewall is formed, the gateinsulating layer 405 might be etched.

Then, impurities are further added to the semiconductor layer to form afirst impurity region 410 right under the insulating layer (sidewall)409 and a second impurity region 411 having a higher concentration ofimpurities than the first impurity region 410. The structure having suchimpurity regions is called an LDD (Lightly Doped Drain) structure. Whenthe first impurity region 410 is overlapped with the gate electrodelayer 406, the structure is called a GOLD (Gate-drain Overlapped LDD)structure.

Then, as shown in FIG. 1C, an insulating layer is formed so as to coverthe semiconductor layer and the gate electrode layer 406. The insulatinglayer is formed using an inorganic material, an organic material, or thelike, which has an insulating property. As an inorganic material havingan insulating property, silicon oxide, silicon nitride, or the like canbe used. As an organic material having an insulating property,polyimide, acrylic, polyamide, polyimide amide, resist,benzocyclobutene, siloxane, or polysilazane can be used. Siloxane is aresin including a bond of silicon (Si) and oxygen (O) and has a skeletonstructure formed by the bond of silicon (Si) and oxygen (O). As asubstituent of the siloxane, an organic group containing at leasthydrogen (for example, an alkyl group or aromatic hydrocarbon) is used.Alternatively, a fluoro group may be used as the substituent. Furtheralternatively, a fluoro group and an organic group containing at leasthydrogen may be used as the substituent. Polysilazane is formed by usinga polymer material having the bond of silicon (Si) and nitrogen (Ni) asa starting material.

FIG. 1C shows a mode where the insulating layer is formed to have astacked-layer structure, and a first insulating layer 414 a, a secondinsulating layer 414 b, and a third insulating layer 414 c are formed inthis order from the bottom. The first insulating layer 414 a ispreferably formed by a plasma CVD method so as to contain much hydrogensince dangling bonds of the semiconductor layer can be reduced byhydrogen.

The second insulating layer 414 b is preferably formed using an organicmaterial since planarity can be improved. The third insulating layer 414c is preferably formed using an inorganic material in order to preventdischarge of moisture or the like from the second insulating layer 414 bformed of an organic material or to prevent intrusion of moisturethrough the second insulating layer 414 b.

Then, a contact hole is formed in the insulating layer to expose thesecond impurity region 411, and as shown in FIG. 1D, a conductive layer415 is formed so as to fill the contact hole. The conductive layer 415includes a film formed of an element selected from aluminum (Al),titanium (Ti), molybdenum (Mo), and tungsten (W), an alloy filmincluding such elements, an alloy film including such elements andsilicon, or the like. Further, the conductive layer 415 can be formed tohave a single-layer structure or a stacked-layer structure. After that,the conductive layer 415 is patterned into a desired shape, and a sourceelectrode, a drain electrode, and other electrodes are formed at thesame time.

In order to lower contact resistance between the source electrode andthe drain electrode; and the second impurity region 411, a silicide maybe formed on the impurity region. For example, a film including a metalelement (typically, Ni) is formed on the second impurity region 411 andheated by a thermal annealing method using an annealing furnace, a laserannealing method, or a rapid thermal annealing method (RTA method). As aresult, a silicide including the metal element and silicon is formed onthe second impurity region; therefore, improvement in on current ormobility can be realized.

In this manner, thin film transistors are completed in a control circuitportion 202 and a memory element region 201. In the control circuitportion 202, a circuit (for example, a writing circuit, a readingcircuit, a sense amplifier, an output circuit, a buffer or the like) isformed by using the thin film transistors.

Next, an insulating layer 416 is formed so as to cover the conductivelayer 415. The insulating layer 416 may be formed using an inorganicmaterial, an organic material, or the like which has an insulatingproperty, and may have a single-layer structure or a stacked-layerstructure. An inorganic material or an organic material similar to thoseof the first insulating layer 414 a, the second insulating layer 414 b,and the third insulating layer 414 c can be used.

Then, as shown in FIG. 2A, a contact hole is formed in the insulatinglayer 416 so as to expose the conductive layer 415, and a conductivelayer 417 is formed so as to fill the contact hole. The conductive layer417 may be formed to have a single-layer structure or a stacked-layerstructure. The conductive layer 417 includes a film formed of an elementselected from aluminum (Al), titanium (Ti), molybdenum (Mo), andtungsten (W), an alloy film including such elements, an alloy filmincluding such elements and silicon, or the like. Alternatively, theconductive layer 417 may be formed by using a light transmittingmaterial such as indium tin oxide (ITO), indium tin oxide containingsilicon oxide, or indium oxide containing 2 to 20% of zinc oxide. Afterthat, the conductive layer 417 is patterned into a desired shape. Thepatterned conductive layer 417 can function as a lower electrode of thememory element.

Although this embodiment mode shows the case where the lower electrodeof the memory element is formed of the conductive layer 417, it may beformed of the conductive layer 415. That is, the conductive layer 415 tobe a source electrode or a drain electrode of a thin film transistor maybe used in common for the lower electrode of the memory element.

Next, an insulating layer is formed so as to cover the patternedconductive layer 417, and a plurality of openings are provided. FIG. 2Ashows an example where two openings are provided. A partition wall 418provided with an opening 902 which exposes the conductive layer 417 andcovers an end portion of the conductive layer 417; and an opening 901which exposes the conductive layer 417 and an end portion of theconductive layer 417 is formed. The partition wall 418 may be formedusing an organic material, an inorganic material, or the like. Aninorganic material or an organic material similar to those of the firstinsulating layer 414 a, the second insulating layer 414 b, and the thirdinsulating layer 414 c can be used. A side surface of the opening of thepartition wall 418 is preferably tapered, which can prevent breakage ofa thin film to be formed later.

Next, as shown in FIG. 2B, a material layer 408 of the memory element isformed in the opening of the partition wall. The material layer 408 ofthe memory element can be formed by a vapor deposition method, a spincoating method, or a droplet discharging method typified by an ink-jetmethod.

Since the material layer 408 of the memory element can also be formedusing the same material as an electroluminescent layer included in alight emitting element, the memory element and the light emittingelement can be formed through a common step. As the light emittingelement, an organic EL element using a layer including an organiccompound as the electroluminescent layer or an inorganic EL elementusing an inorganic material as an illuminant can be used. That is, amemory device having a display function can be formed.

Next, a conductive layer to be an opposite electrode 420 is formed.Since the opposite electrode 420 can be formed over the whole surface ofthe memory element region, patterning by a photolithography method isnot required. Needless to say, the opposite electrode 420 may beselectively formed by patterning. The opposite electrode 420 canfunction as an upper electrode of the memory element.

Thus, a memory element 426 including the conductive layer 417, thematerial layer 408 of the memory element, and the opposite electrode 420is formed.

More preferably, an insulating layer 421 functioning as a protectivefilm is formed. In order to improve impact resistance, the insulatinglayer 421 is preferably formed thick. Therefore, the insulating layer421 is preferably formed using an organic material such as an epoxyresin or a polyimide resin. In addition, a drying agent is preferablydispersed in the insulating layer 421 in order to provide a hygroscopicproperty. This is because intrusion of moisture can be preventedparticularly in the case where the material layer of the memory elementis formed using an organic material. By sealing with the insulatinglayer 421 in this manner, intrusion of unnecessary oxygen as well asmoisture can be prevented.

In this manner, a circuit including the thin film transistors which areprovided in the control circuit portion 202 can be formed, and thememory element 426 which is formed over the same substrate as thecircuit through a common step to that of the circuit and provided in thememory element region 201, and a thin film transistor connected to thememory element 426 can be formed. The memory element is controlled bythe thin film transistor. In this manner, a mode where a thin filmtransistor is connected to a memory element is called an active matrixtype.

In the memory device of the invention, the memory element 426 and thecontrol circuit can be formed over the same substrate through a commonstep; therefore, manufacturing cost can be reduced. Further, since astep of mounting a memory element formed by a conventional IC is notrequired, a connection defect with the control circuit can be prevented.

FIG. 3 shows a mode where an antenna 430 for supplying power or the liketo the memory element 426 is provided. This embodiment mode shows a modewhere the antenna 430 is formed in the opening provided in the partitionwall.

The antenna 430 can be formed so as to be connected to an electrode 419which is electrically connected to a thin film transistor provided inthe memory element region 201. As a conductive material for the antenna,an element selected from aluminum (Al), titanium (Ti), silver (Ag),copper (Cu), gold (Au), platinum (Pt), nickel (Ni), palladium (Pd),tantalum (Ta), and molybdenum (Mo), or an alloy material or a compoundmaterial including such elements as a main component can be used. Theconductive layer is formed to have a single-layer structure or astacked-layer structure. As the conductive material for the antenna, alow-resistance material such as Cu (copper), Ag (silver), or Al(aluminum) is preferably used. Further, the antenna 430 is preferablyformed to be thick in order to lower the resistance thereof. The antenna430 can be formed by a vapor deposition method, a printing method, aplating method, or a droplet discharging method typified by an ink-jetmethod.

By forming the antenna 430 and the thin film transistor over the samesubstrate in this manner, wireless communication with a reader/writerdevice can be performed. As a result, multivalued information can beobtained from the memory element 426 without breakdown. For example,when an electromagnetic coupling method or an electromagnetic inductionmethod (for example, a 13.56 MHz band) is employed as a signaltransmission method of the semiconductor device, electromagneticinduction caused by a change in magnetic field density is used.Therefore, the conductive layer functioning as an antenna is formed inan annular shape (for example, a loop antenna) or a spiral shape (forexample, a spiral antenna). Further, when a microwave method (forexample, a UHF band (860 to 960 MHz band), a 2.45 GHz band, or the like)is employed as the signal transmission method of the semiconductordevice, the shape such as a length of the conductive layer functioningas the antenna may be set as appropriate in consideration of awavelength of an electromagnetic wave used for signal transmission. Forexample, the conductive layer functioning as the antenna can be formedin a linear shape (for example, a dipole antenna), a flat shape (forexample, a patch antenna), a ribbon-like shape, or the like. The shapeof the conductive layer functioning as the antenna is not limited to alinear shape, and the conductive layer functioning as the antenna may beformed in a curved-line shape, a meandering shape, or a combinationthereof, in consideration of a wavelength of an electromagnetic wave.

Although a semiconductor device provided with the memory element regionand the antenna can be completed through the aforementioned steps, agroove may be formed thereafter as shown in FIG. 4A, and an etchant 441may be introduced into the groove so that the glass substrate 401 ispeeled off. At this time, a resin substrate 440 attached onto theinsulating layer 421 is preferably used as a supporting base in order toeasily peel off the glass substrate 401. Note that the resin substrate440 may be attached by using an adhesion function of the insulatinglayer 421. The resin substrate 440 may be formed using plastic typifiedby polyethylene terephthalate (PET), polyethylene naphthalate (PEN), orpolyethersulfone (PES); or a synthetic resin such as acrylic. Since sucha resin substrate is quite thin, it has flexibility. Therefore, theglass substrate 401 can be sequentially peeled off by attaching therolled up resin substrate 440 onto the insulating layer 421. Such a stepis suitable for mass production.

The etchant 441 is not specifically limited to a certain type as long asthe separation layer 402 can be selectively etched. For example, ahalogen compound may be used. When amorphous silicon or tungsten is usedfor the separation layer, ClF₃ (chlorine trifluoride) can be used as theetchant. In addition, when silicon oxide is used for the separationlayer, HF (hydrogen fluoride) can be used as the etchant.

The invention is not limited to a separation method where a separationlayer is selectively etched by an etchant, and other known separationmethods may be employed as well. For example, a metal oxide film (forexample, a tungsten oxide film or a molybdenum oxide film) may beprovided between an integrated circuit and a substrate having high heatresistance, and after the metal oxide film is weakened, the integratedcircuit including a TFT provided over the metal oxide film can bepeeled. Alternatively, at least a part of a separation layer may bedestroyed by irradiation with laser light, so that the integratedcircuit including a TFT can be peeled off from a substrate.

Then, as shown in FIG. 4B, a resin substrate 442 is attached instead ofthe peeled glass substrate 401. Note that the resin substrate 442 may beformed using a material similar to that of the resin substrate 440.

As a result of peeling off the glass substrate 401 in this manner,reduction in thickness and weight of a semiconductor device includingthe memory element can be achieved and flexibility and impact resistancethereof can be improved.

Further, the substrate is divided into a plurality of semiconductordevices each including a memory element. As a result, cost reduction ofthe semiconductor device including the memory element can be achieved.

Further, a protective layer such as a gas barrier layer may be providedon each surface of the resin substrates 440 and 442. The provision ofthe protective layer can prevent intrusion of oxygen and alkalineelements, and thereby reliability can be improved. The protective layeris formed using an inorganic material containing nitrogen, such as analuminum nitride film or a silicon nitride film.

Although this embodiment mode shows a mode where the glass substrate 401is removed and the resin substrates 440 and 442 are attached, theinvention is not limited to this. Note that reduction in weight andthickness of the semiconductor device including the memory element canbe achieved by removing the glass substrate 401.

Although a thin film transistor in this embodiment mode has astacked-layer structure where a semiconductor layer, a gate insulatinglayer, and a gate electrode layer are sequentially stacked over asubstrate, the thin film transistor used in the invention is not limitedto have this structure, and such a structure may be adopted, in which agate electrode layer, an insulating layer, and a semiconductor layer arestacked in this order. Further, although the thin film transistor hasimpurity regions such as the first impurity region (also referred to asa low concentration impurity region) 410 and the second impurity region(also referred to as a high concentration impurity region) 411, theinvention is not limited to these, and a single-drain structure having auniform concentration of impurities may be adopted as well.

In addition, a multilayer structure may be adopted where a plurality ofthin film transistors shown in this embodiment mode are stacked. Whensuch a multilayer structure is employed, a low dielectric constant(low-k) material is preferably used as a material of an insulating layerin order to reduce parasitic capacitance which is generated in theinsulating layer between the stacked thin film transistors. For example,in addition to the aforementioned materials, a resin material such as anepoxy resin or an acrylic resin, or an organic material such as siloxanemay be used. By employing the multilayer structure which reducesparasitic capacitance, reduction of area, high-speed operation and lowpower consumption of the memory device can be achieved.

In this manner, in the invention, a memory cell can be multivalued inone memory cell. Thus, storage capacity of a memory region of thesemiconductor device can be increased.

Embodiment Mode 2

In this embodiment mode, a method of forming a memory element over aglass substrate as an insulating substrate is described. An electrode isformed of a stacked film and the memory element is formed over aplurality of electrode steps. Note that a mode in which the memoryelement and a circuit (a control circuit) for controlling the memoryelement are formed over the same substrate through a common step isshown. In addition, parts of the steps same as Embodiment Mode 1 aredescribed using the same drawings and the same reference numerals.

First, in the same manner as in FIG. 1A, the separation layer 402 isformed over the glass substrate 401. Quartz or the like is used for theinsulating substrate as well as glass. The separation layer 402 isobtained by forming a film containing metal or a film containing siliconentirely or selectively over the substrate.

Next, in the same manner as in Embodiment Mode 1, the insulating layer403 is formed so as to cover the separation layer 402. The insulatinglayer 403 is formed of silicon oxide, silicon nitride, or the like.Then, the semiconductor layer is formed over the insulating layer 403and crystallized by laser crystallization, thermal crystallization usinga metal catalyst, or the like, and then patterned into a desired shapeto form the island-shaped semiconductor layer. The laser crystallizationmay be performed by using a continuous wave laser or a pulsed laser.

Next, in the same manner as in Embodiment Mode 1, the gate insulatinglayer 405 is formed so as to cover the semiconductor layer. The gateinsulating layer 405 is formed using silicon oxide, silicon nitride, orthe like. The gate insulating layer 405 can be formed by a CVD method, athermal oxidation method, or the like. Alternatively, the semiconductorlayer and the gate insulating layer 405 may be continuously formed by aCVD method, and subsequently can be patterned at the same time. In thiscase, impurity contamination at the boundary between each layer can besuppressed.

Then, in the same manner as in Embodiment Mode 1, the gate electrodelayer 406 is formed. The gate electrode layer 406 is formed using anelement selected from tantalum (Ta), tungsten (W), titanium (Ti),molybdenum (Mo), aluminum (Al), and copper (Cu), or an alloy material ora compound material including such elements as a main component, andpatterned into a desired shape. In the case of patterning by aphotolithography method, the width of the gate electrode can be madenarrower by using a resist mask of which width is narrowed by plasmaetching or the like. Accordingly, performance of a transistor can beimproved. The gate electrode layer 406 may have either a single-layerstructure or a stacked-layer structure.

Next, in the same manner as in Embodiment Mode 1, impurity elementswhich imparts a conductivity type are added to the semiconductor layerto form the impurity region 407. The impurity region 407 is formed byusing a resist mask formed by a photolithography method and adding animpurity element such as phosphorus, arsenic, or boron. With theimpurity element, a polarity of either an n-channel type or a p-channeltype can be determined.

Then, in the same manner as in Embodiment Mode 1, as shown in FIG. 1B,the insulating layer is formed of an insulator containing silicon, forexample, silicon nitride, and the insulating layer is anisotropically(perpendicularly) etched to form the insulating layer (also referred toas the sidewall) 409 which contacts with a side surface of the gateelectrode. When the sidewall is formed, the gate insulating layer 405might be etched.

Next, in the same manner as in Embodiment Mode 1, impurities are furtheradded to the semiconductor layer to form the first impurity region 410right under the insulating layer (sidewall) 409 and the second impurityregion 411 having a higher concentration of impurities than the firstimpurity region 410.

Then, in the same manner as in Embodiment Mode 1, the insulating layeris formed so as to cover the semiconductor layer and the gate electrodelayer 406. The insulating layer is formed using an inorganic material,an organic material, or the like, which has an insulating property. Asan inorganic material having an insulating property, silicon oxide,silicon nitride, or the like can be used. As an organic material havingan insulating property, polyimide, acrylic, polyamide, polyimide amide,resist, benzocyclobutene, siloxane, or polysilazane can be used.

Here, similarly to FIG. 1C, a mode where the insulating layer is formedto have a stacked-layer structure is shown, and the first insulatinglayer 414 a, the second insulating layer 414 b, and the third insulatinglayer 414 c are formed in this order from the bottom. The firstinsulating layer 414 a is preferably formed by a plasma CVD method so asto contain much hydrogen since dangling bonds of the semiconductor layercan be reduced by hydrogen. The second insulating layer 414 b ispreferably formed using an organic material since planarity can beimproved. The third insulating layer 414 c is preferably formed using aninorganic material in order to prevent discharge of moisture or the likefrom the second insulating layer 414 b formed of an organic material orto prevent intrusion of moisture through the second insulating layer 414b.

Next, the contact hole is formed in the insulating layer to expose thesecond impurity region 411, and similarly to FIG. 1D, the conductivelayer 415 is formed so as to fill the contact hole. The conductive layer415 includes a film formed of an element selected from aluminum (Al),titanium (Ti), molybdenum (Mo), and tungsten (W), an alloy filmincluding such elements, an alloy film including such elements andsilicon, or the like. Further, the conductive layer 415 can be formed tohave a single-layer structure or a stacked-layer structure. After that,the conductive layer 415 is patterned into a desired shape, and a sourceelectrode, a drain electrode, and other electrodes are formed at thesame time.

In order to lower contact resistance between the source electrode andthe drain electrode; and the second impurity region 411, a silicide maybe formed on the impurity region. For example, a film including a metalelement (typically, Ni) is formed on the second impurity region 411 andheated by a thermal annealing method using an annealing furnace, a laserannealing method, or a rapid thermal annealing method (RTA method). As aresult, a silicide including the metal element and silicon is formed onthe second impurity region; therefore, improvement in on current ormobility can be realized.

In this manner, the thin film transistors are completed in the controlcircuit portion 202 and the memory element region 201. In the controlcircuit portion 202, a circuit is formed by using the thin filmtransistors.

Next, in the same manner as in Embodiment Mode 1, the insulating layer416 is formed so as to cover the conductive layer 415. The insulatinglayer 416 may be formed using an inorganic material, an organicmaterial, or the like which has an insulating property, and may have asingle-layer structure or a stacked-layer structure. An inorganicmaterial or an organic material similar to those of the first insulatinglayer 414 a, the second insulating layer 414 b, and the third insulatinglayer 414 c can be used for the insulating layer 416.

After that, as shown in FIG. 5A, the insulating layer 416 is selectivelyetched to form a contact hole so that the conductive layer 415 isexposed, and conductive layers 903 and 904 are stacked so as to fill thecontact hole. Each of the conductive layers 903 and 904 includes a filmformed of an element selected from aluminum (Al), titanium (Ti),molybdenum (Mo), and tungsten (W), an alloy film including suchelements, an alloy film including such elements and silicon, or thelike. Alternatively, the conductive layers 903 and 904 may be formed byusing a light transmitting material such as indium tin oxide (ITO),indium tin oxide containing silicon oxide, or indium oxide containing 2to 20% of zinc oxide. In FIG. 5A, titanium is used for the conductivelayer 903 and aluminum is used for the conductive layer 904. Theconductive layer 904 is made thicker than the conductive layer 903, sothat two steps having different height can be provided. The method isdescribed below.

As shown in FIG. 51B, the conductive layers 903 and 904 are processedinto a desired shape. The conductive layers 903 and 904 are processed soas to expose a surface of the conductive layer 904. The conductive layer904 is made thicker than the conductive layer 903, so that two stepshaving different height are provided. It is considered that the higher astep is, the lower a breakdown voltage of the memory element is.Therefore, by providing two steps having different height with the useof the lower electrode, two memories having a different breakdownvoltage of the memory element can be formed. That is, the conductivelayers 903 and 904 can function as the lower electrodes of the memoryelement and also as the steps for adjusting the breakdown voltage of thememory element.

Next, as shown in FIG. 6A, an insulating layer is formed so as to coverthe conductive layers 903 and 904, and a plurality of openings areprovided. The partition wall 418 provided with openings 905, 906, and907 is formed.

As described above, a plurality of openings and the conductive layerhaving a plurality of steps can be formed.

Note that although this embodiment mode shows the case where the lowerelectrode of the memory element is formed of the conductive layers 903and 904, the conductive layer 415 to be a source electrode or a drainelectrode of a thin film transistor may be used in common for the lowerelectrode of the memory element.

Next, as shown in FIG. 6B, the material layer 408 of the memory elementis formed in the opening of the partition wall. The material layer 408of the memory element can be formed by a vapor deposition method, a spincoating method, or a droplet discharging method typified by an ink-jetmethod.

Since the material layer 408 of the memory element can also be formedusing the same material as the electroluminescent layer included in thelight emitting element, the memory element and the light emittingelement can be formed through a common step. That is, a memory devicehaving a display function can be formed.

Next, the conductive layer to be the opposite electrode 420 is formed.Since the opposite electrode 420 can be formed over the whole surface ofthe memory element region, patterning by a photolithography method isnot required. Needless to say, the opposite electrode 420 may beselectively formed by patterning. The opposite electrode 420 canfunction as the upper electrode of the memory element.

Thus, the memory element 426 including the conductive layer 417, thematerial layer 408 of the memory element, and the opposite electrode 420is formed. In one memory cell, three memory elements corresponding tothe three openings 905, 906, and 907 are formed, and this memory cellhas three voltage values by which electric resistance is changed. Thisvoltage value by which electric resistance is changed corresponds to areading voltage value (or a reading current value) or a writing voltagevalue (or a writing current value).

Change of a reading current value of a manufactured memory having aplurality of openings is described in detail by using formulae. Aresistance value of the material layer in the memory before a shortcircuit is denoted by Ra. Contact resistance of the opposite electrodeand the lower electrode after the short circuit is denoted by R1, R2,and R3 with respect to the openings 905, 906, and 907, respectively. Avoltage applied to the memory element when reading is performed isdenoted by Vr. A reading current value before writing is expressed byFormula 1.

$\begin{matrix}{{I\; 0} = {{\frac{Vr}{\left( {{Ra} + {R\; 1}} \right)} + \frac{Vr}{\left( {{Ra} + {R\; 2}} \right)} + \frac{Vr}{\left( {{Ra} + {R\; 3}} \right)}} \approx \frac{3\;{Vr}}{Ra}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$Note that approximation is performed on the assumption that Ra>>R1, R2,R3 is satisfied. When first writing is preformed, a short circuit iscaused between the opposite electrode and the lower electrode at theopening 906. A reading current value I1 after the short circuit isexpressed by Formula 2.

$\begin{matrix}{{I\; 1} = {{\frac{Vr}{\left( {{Ra} + {R\; 1}} \right)} + \frac{Vr}{R\; 2} + \frac{Vr}{\left( {{Ra} + {R\; 3}} \right)}} \approx \frac{Vr}{R\; 2}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$Note that approximation is performed on the assumption that Ra>>R1, R2,R3 is satisfied. At this time, a ratio of the current values before andafter the first writing is expressed by Formula 3.

$\begin{matrix}{\frac{I\; 1}{I\; 0} = {{\frac{Vr}{R\; 2} \times \frac{Ra}{3\;{Vr}}} = \frac{Ra}{3\; R\; 2}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$Next, when second writing is performed, a short circuit is causedbetween the opposite electrode and the lower electrode at the opening905. A reading current value I2 after the short circuit is expressed byFormula 4.

$\begin{matrix}{{I\; 2} = {{\frac{Vr}{R\; 1} + \frac{Vr}{R\; 2} + \frac{Vr}{\left( {{Ra} + {R\; 3}} \right)}} \approx {\frac{Vr}{R\; 1} + \frac{Vr}{R\; 2}}}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$At this time, a ratio of the current values before and after the secondwriting is expressed by Formula 5.

$\begin{matrix}{\frac{I\; 2}{I\; 1} = {{\left( {\frac{Vr}{R\; 1} + \frac{Vr}{R\; 2}} \right) \times \frac{R\; 2}{Vr}} = {\frac{{R\; 1} + {R\; 2}}{R\; 1} = {1 + \frac{R\; 2}{R\; 1}}}}} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack\end{matrix}$Next, when third writing is performed, a short circuit is caused betweenthe opposite electrode and the lower electrode at the opening 907. Areading current value I3 after the short circuit is expressed by Formula6.

$\begin{matrix}{{I\; 3} = {\frac{Vr}{R\; 1} + \frac{Vr}{R\; 2} + \frac{Vr}{R\; 3}}} & \left\lbrack {{Formula}\mspace{14mu} 6} \right\rbrack\end{matrix}$At this time, a ratio of the current values before and after the thirdwriting is expressed by Formula 7.

$\begin{matrix}{\frac{I\; 3}{I\; 2} = {{\left( {\frac{Vr}{R\; 1} + \frac{Vr}{R\; 2} + \frac{Vr}{R\; 3}} \right) \div \left( {\frac{Vr}{R\; 1} + \frac{Vr}{R\; 2}} \right)}=={1 + \frac{R\; 1R\; 2}{R\; 3\left( {{R\; 1} + {R\; 2}} \right)}}}} & \left\lbrack {{Formula}\mspace{14mu} 7} \right\rbrack\end{matrix}$According to Formula 5, a relation of R2>R1 needs to be satisfied inorder to increase the ratio of the current values before and afterwriting. For example, the following method can be considered.

FIG. 13A shows a top view of a memory element and a thin film transistorwhich are in process of manufacture. FIG. 13B shows a cross sectionalview along a dotted line AB in FIG. 13A. The thin film transistorincludes the gate electrode layer 406, an island-shaped semiconductorlayer 404, and the conductive layers 415 functioning as a sourceelectrode or a drain electrode. The conductive layers 415 areelectrically connected to the island-shaped semiconductor layer 404through contact holes 919 and 920 which are formed in the firstinsulating layer 414 a, the second insulating layer 414 b, and the thirdinsulating layer 414 c. In addition, one of the conductive layers 415 iselectrically connected to the conductive layer 903 through a contacthole 921 formed in the insulating layer 416.

The conductive layer 904 is formed over the conductive layer 903. Asshown in FIG. 13A, the area of the conductive layer 903 is made largerthan that of the conductive layer 904.

An end face of the conductive layer 903 (namely, a first step) isexposed in the opening 905 of the partition wall 418. An end face of theconductive layer 904 (namely, a second step) is exposed in the opening906 of the partition wall 418. The second step is higher than the firststep. A top surface of the conductive layer 904 is exposed in theopening 907 of the partition wall 418 and a step is not formed in theopening 907. Note that the opening 905, 906, 907 may also be referred toas regions surrounded by portions of the partition wall 418.

FIG. 13B shows a state through a step same as the cross sectional viewshown in FIG. 6A. Subsequently, a material layer of the memory elementis formed over the openings 905, 906, and 907, and the conductive layeris formed thereover; therefore, a memory element and a thin filmtransistor shown in FIG. 6B are manufactured. For example, by an ink-jetmethod, a material droplet to be the material layer of the memoryelement is dropped in the openings 905, 906, and 907 which aresurrounded by the partition wall 418.

Since contact resistance is proportional to the area of the opening, bymaking the area of the opening 905 larger than that of the opening 906,a ratio of contact resistance can be increased and the ratio of thecurrent value before and after the second writing can be increased.

In addition, as shown in a top view of FIG. 13C, it is effective todevise a shape of the opening in order to further increase the area ofthe opening. FIG. 13C shows an example where positions and shapes of theopenings 905, 906, and 907 of the partition wall 418 are devised. Theopening 905 in FIG. 13C is larger than the opening 905 in FIG. 13A. Theopening 907 in FIG. 13C is larger than the opening 907 in FIG. 13A.Further, the openings 905, 906, and 907 in FIG. 13A are arranged in aline, while the openings 905, 906, and 907 in FIG. 13C are not arrangedin a line. As shown in FIG. 13C, the position of the openings is notlimited, and the openings can be freely provided.

After that, the insulating layer 421 functioning as a protective film isformed. In order to improve impact resistance, the insulating layer 421is preferably formed thick. Therefore, the insulating layer 421 ispreferably formed using an organic material such as an epoxy resin or apolyimide resin. In addition, a drying agent is preferably dispersed inthe insulating layer 421 in order to provide a hygroscopic property.This is because intrusion of moisture can be prevented particularly inthe case where the material layer of the memory element is formed usingan organic material. By sealing with the insulating layer 421 in thismanner, intrusion of unnecessary oxygen as well as moisture can beprevented.

In this manner, the circuit including the thin film transistors whichare provided in the control circuit portion 202 can be formed; and thememory element 426 which is formed over the same substrate as thecircuit and provided in the memory element region 201, and the thin filmtransistor connected to the memory element 426 can be formed.

In the semiconductor device of the invention, the memory element 426 andthe control circuit can be formed over the same substrate; therefore,manufacturing cost can be reduced. Further, since a conventional step ofmounting a memory element formed by an IC is not required, a connectiondefect with the control circuit can be prevented.

FIG. 7 shows a mode where the antenna 430 for supplying power or thelike to the memory element 426 is provided. This embodiment mode shows amode where the antenna 430 is formed in the opening provided in thepartition wall.

The antenna 430 can be formed to be connected to the thin filmtransistor provided in the memory element region 201 and is formed of aconductive material, preferably a low-resistance material such as Cu(copper), Ag (silver), or Al (aluminum). Further, the antenna 430 ispreferably formed to be thick in order to lower the resistance thereof.The antenna 430 can be formed by a vapor deposition method, a printingmethod, a plating method, or a droplet discharging method typified by anink-jet method.

By forming the antenna 430 and the circuit over the same substrate inthis manner, wireless communication with a reader/writer device can beperformed. As a result, multivalued information can be obtained from thememory element 426 without breakdown.

Although a memory device can be completed through the aforementionedsteps, a groove may be formed thereafter as shown in FIG. 8A, and theetchant 441 may be introduced into the groove so that the glasssubstrate 401 is peeled off. At this time, the resin substrate 440attached onto the insulating layer 421 is preferably used as asupporting base in order to easily peel off the glass substrate 401.Note that the resin substrate 440 may be attached by using an adhesionfunction of the insulating layer 421. The resin substrate 440 may beformed using plastic typified by polyethylene terephthalate (PET),polyethylene naphthalate (PEN), or polyethersulfone (PES); or asynthetic resin such as acrylic. Since such a resin substrate is quitethin, it has flexibility. Therefore, the glass substrate 401 can besequentially peeled off by attaching the rolled up resin substrate 440onto the insulating layer 421. Such a step is suitable for massproduction.

The etchant 441 is not specifically limited to a certain type as long asthe separation layer 402 can be selectively etched. For example, ahalogen compound may be used. When amorphous silicon or tungsten is usedfor the separation layer, ClF₃ (chlorine trifluoride) can be used as theetchant. In addition, when silicon oxide is used for the separationlayer, HF (hydrogen fluoride) can be used as the etchant.

Then, as shown in FIG. 8B, the resin substrate 442 is attached insteadof the peeled glass substrate 401. Note that the resin substrate 442 maybe formed using a material similar to that of the resin substrate 440.

As a result of peeling off the glass substrate 401 in this manner,reduction in thickness and weight of a semiconductor device includingthe memory element and the antenna can be achieved and flexibility andimpact resistance thereof can be improved.

Further, the substrate is divided into a plurality of semiconductordevices each including a memory element. As a result, cost reduction ofthe semiconductor device including the memory element can be achieved.

Further, a protective layer such as a gas barrier layer may be providedon each surface of the resin substrates 440 and 442. The provision ofthe protective layer can prevent intrusion of oxygen and alkalineelements, and thereby reliability can be improved. The protective layeris formed using an inorganic material containing nitrogen, such as analuminum nitride film or a silicon nitride film.

Although this embodiment mode shows a mode where the glass substrate 401is removed and the resin substrates 440 and 442 are attached, theinvention is not limited to this. Note that reduction in weight andthickness of the semiconductor device including the memory element canbe achieved by removing the glass substrate 401.

Although a thin film transistor in this embodiment mode has astacked-layer structure where a semiconductor layer, a gate insulatinglayer and a gate electrode layer are sequentially stacked over asubstrate, the thin film transistor used in the invention is not limitedto have this structure, and such a structure may be adopted, in which agate electrode layer, an insulating layer, and a semiconductor layer arestacked in this order. Further, although the thin film transistor hasimpurity regions such as the first impurity region (also referred to asa low concentration impurity region) 410 and the second impurity region(also referred to as a high concentration impurity region) 411, theinvention is not limited to these, and a single-drain structure having auniform concentration of impurities may be adopted as well.

In addition, a multilayer structure may be adopted where a plurality ofthin film transistors shown in this embodiment mode are stacked. Whensuch a multilayer structure is employed, a low dielectric constant(low-k) material is preferably used as a material of an insulating layerin order to reduce parasitic capacitance which is generated in theinsulating layer between the stacked thin film transistors. For example,in addition to the aforementioned materials, a resin material such as anepoxy resin or an acrylic resin, or an organic material such as siloxanemay be used. By employing the multilayer structure which reducesparasitic capacitance, reduction of area, high-speed operation and lowpower consumption of the memory device can be achieved.

In this manner, in the invention, a memory cell can be multivalued inone memory cell. Thus, storage capacity of a memory region in thesemiconductor device can be increased.

Note that this embodiment mode can be freely implemented in combinationwith Embodiment Mode 1.

Embodiment Mode 3

In this embodiment mode, description is made of a method in which amargin of a reading current is increased by using difference of contactresistance with that of the opposite electrode for each region when aplurality of regions having a different breakdown voltage (writingvoltage value) are formed in a memory cell. In addition, parts of thesteps same as those of Embodiment Mode 1 are described using the samedrawings and the same reference numerals.

When a plurality of regions having a different breakdown voltage areformed in the memory cell, a lower electrode is formed of a differentmaterial for each region so that a conductive layer with high contactresistance with an upper electrode is used for a region with a lowbreakdown voltage and a conductive layer with low contact resistance isused for a region with a high breakdown voltage. Accordingly, a ratio ofreading current between bits can be increased, which is effective. Themethod is described below.

First, in the same manner as in FIG. 1A, the separation layer 402 isformed over the glass substrate 401. Quartz, silicon, metal, or the likeis used for the insulating substrate as well as glass. The separationlayer 402 is obtained by forming a film containing metal or a filmcontaining silicon entirely or selectively over the substrate.

Next, in the same manner as in Embodiment Mode 1, the insulating layer403 is formed so as to cover the separation layer 402. The insulatinglayer 403 is formed of silicon oxide, silicon nitride, or the like.Then, the semiconductor layer is formed over the insulating layer 403and crystallized by laser crystallization, thermal crystallization usinga metal catalyst, or the like, and then patterned into a desired shapeto form the island-shaped semiconductor layer. The laser crystallizationmay be performed by using a continuous wave laser or a pulsed laser.

Next, in the same manner as in Embodiment Mode 1, the gate insulatinglayer 405 is formed so as to cover the semiconductor layer. The gateinsulating layer 405 is formed using silicon oxide, silicon nitride, orthe like. The gate insulating layer 405 can be formed by a CVD method, athermal oxidation method, or the like. Alternatively, the semiconductorlayer and the gate insulating layer 405 may be continuously formed by aCVD method, and subsequently can be patterned at the same time. In thiscase, impurity contamination at the boundary between each layer can besuppressed.

Then, in the same manner as in Embodiment Mode 1, the gate electrodelayer 406 is formed. The gate electrode layer 406 is formed by using anelement selected from tantalum (Ta), tungsten (W), titanium (Ti),molybdenum (Mo), aluminum (Al), and copper (Cu), or an alloy material ora compound material including such elements as a main component, andpatterned into a desired shape. In the case of patterning by aphotolithography method, the width of the gate electrode can be madenarrower by using a resist mask of which width is narrowed by plasmaetching or the like. Accordingly, performance of a transistor can beimproved. The gate electrode layer 406 may have either a single-layerstructure or a stacked-layer structure.

Next, in the same manner as in Embodiment Mode 1, impurity elementswhich imparts a conductivity type are added to the semiconductor layerto form the impurity region 407. The impurity region 407 is formed byusing a resist mask formed by a photolithography method and adding animpurity element such as phosphorus, arsenic, or boron. With theimpurity element, a polarity of either an n-channel type or a p-channeltype can be determined.

Then, in the same manner as in Embodiment Mode 1, as shown in FIG. 1B,the insulating layer is formed of an insulator containing silicon, forexample, silicon nitride, and the insulating layer is anisotropically(perpendicularly) etched to form the insulating layer (also referred toas the sidewall) 409 which contacts with a side surface of the gateelectrode. When the sidewall is formed, the gate insulating layer 405might be etched.

Next, in the same manner as in Embodiment Mode 1, impurities are furtheradded to the semiconductor layer to form the first impurity region 410right under the insulating layer (sidewall) 409 and the second impurityregion 411 having a higher concentration of impurities than the firstimpurity region 410.

Then, in the same manner as in Embodiment Mode 1, the insulating layeris formed so as to cover the semiconductor layer and the gate electrodelayer 406. The insulating layer is formed by using an inorganicmaterial, an organic material, or the like which has an insulatingproperty. As an inorganic material having an insulating property,silicon oxide, silicon nitride, or the like can be used. As an organicmaterial having an insulating property, polyimide, acrylic, polyamide,polyimide amide, resist, benzocyclobutene, siloxane, or polysilazane canbe used.

Here, similarly to FIG. 1C, a mode where the insulating layer is formedto have a stacked-layer structure is shown, and the first insulatinglayer 414 a, the second insulating layer 414 b, and the third insulatinglayer 414 c are formed in this order from the bottom. The firstinsulating layer 414 a is preferably formed by a plasma CVD method so asto contain much hydrogen since dangling bonds of the semiconductor layercan be reduced by hydrogen. The second insulating layer 414 b ispreferably formed using an organic material since planarity can beimproved. The third insulating layer 414 c is preferably formed using aninorganic material in order to prevent discharge of moisture or the likefrom the second insulating layer 414 b formed of an organic material orto prevent intrusion of moisture through the second insulating layer 414b.

Next, the contact hole is formed in the insulating layer to expose thesecond impurity region 411, and similarly to FIG. 1D, the conductivelayer 415 is formed so as to fill the contact hole. The conductive layer415 includes a film formed of an element selected from aluminum (Al),titanium (Ti), molybdenum (Mo), and tungsten (W), an alloy filmincluding such elements, an alloy film including such elements andsilicon, or the like. Further, the conductive layer 415 can be formed tohave a single-layer structure or a stacked-layer structure. After that,the conductive layer 415 is patterned into a desired shape, and a sourceelectrode, a drain electrode, and other electrodes are formed at thesame time.

In order to lower contact resistance between the source electrode andthe drain electrode; and the second impurity region 411, a silicide maybe formed on the impurity region. For example, a film including a metalelement (typically, Ni) is formed on the second impurity region 411 andheated by a thermal annealing method using an annealing furnace, a laserannealing method, or a rapid thermal annealing method (RTA method). As aresult, a silicide including the metal element and silicon is formed onthe second impurity region; therefore, improvement in on current ormobility can be realized.

In this manner, the thin film transistors are completed in the controlcircuit portion 202 and the memory element region 201. In the controlcircuit portion 202, the circuit is formed by using the thin filmtransistors.

Next, the insulating layer 416 is formed so as to cover the conductivelayer 415. The insulating layer 416 may be formed using an inorganicmaterial, an organic material, or the like which has an insulatingproperty, and may have a single-layer structure or a stacked-layerstructure. An inorganic material or an organic material similar to thoseof the first insulating layer 414 a, the second insulating layer 414 b,and the third insulating layer 414 c can be used for the insulatinglayer 416.

As shown in FIG. 9A, the insulating layer 416 is selectively etched toform a contact hole so that the conductive layer 415 is exposed, andconductive layers 911, 912, and 913 are stacked so as to fill thecontact hole. Each of the conductive layers 911, 912, and 913 includes afilm formed of an element selected from aluminum (Al), titanium (Ti),molybdenum (Mo), and tungsten (W), an alloy film including suchelements, an alloy film including such elements and silicon, or thelike. Alternatively, the conductive layers 911, 912, and 913 may beformed by using a light transmitting material such as indium tin oxide(ITO), indium tin oxide containing silicon oxide, or indium oxidecontaining 2 to 20% of zinc oxide.

When contact resistance of the conductive layer 912 and the oppositeelectrode 420 after a short circuit is denoted by R1, contact resistanceof the conductive layer 913 and the opposite electrode 420 after a shortcircuit is denoted by R2, and contact resistance of the conductive layer911 and the opposite electrode 420 after a short circuit is denoted byR3, it is important to select the conductive layers 911 to 913 which cansatisfy R2>R1>R3. The reason is described below. In FIG. 9A, indium tinoxide (ITO) is used for the conductive layer 911, tungsten (W) is usedfor the conductive layer 912, and titanium (Ti) is used for theconductive layer 913.

The conductive layers 911, 912, and 913 are processed into a desiredshape. The conductive layers 911, 912, and 913 can function as the lowerelectrodes of the memory element and also as the steps for adjusting thebreakdown voltage of the memory element.

Next, an insulating layer is formed so as to cover the conductive layers911, 912, and 913, and a plurality of openings are provided. Thepartition wall 418 provided with openings 914, 915, and 916 is formed.

Note that although this embodiment mode shows the case where the lowerelectrode of the memory element is formed of the conductive layers 911to 913, the conductive layer 415 to be a source electrode or a drainelectrode of a thin film transistor may be used in common for the lowerelectrode of the memory element.

Next, as shown in FIG. 9B, the material layer 408 of the memory elementis formed in the opening of the partition wall. The material layer 408of the memory element can be formed by a vapor deposition method, a spincoating method, or a droplet discharging method typified by an ink-jetmethod.

Since the material layer 408 of the memory element can also be formedusing the same material as the electroluminescent layer included in thelight emitting element, the memory element and the light emittingelement can be formed over the same substrate. That is, a memory devicehaving a display function can be formed.

Next, the conductive layer to be the opposite electrode 420 is formed.Since the opposite electrode 420 can be formed over the whole surface ofthe memory element region, patterning by a photolithography method isnot required. Needless to say, the opposite electrode 420 may beselectively formed by patterning. The opposite electrode 420 canfunction as the upper electrode of the memory element.

Thus, the memory element 426 including the conductive layer 417, thematerial layer 408 of the memory element, and the opposite electrode 420is formed. In one memory cell, three memory elements corresponding tothe three openings 914, 915, and 916 are formed, and this memory cellhas three voltage values by which electric resistance is changed.

Writing includes first writing, second writing, and third writing inorder of increasing writing voltage. When the first writing isperformed, a short circuit between the upper electrode and the lowerelectrode is caused in a memory provided in the opening 915 which hasthe highest electrode step. In the case where a semiconductor or aninsulator is used as a memory layer, a current flowing to the memoryprovided in the opening 915 is extremely larger than a current flowingto memories provided in the openings 914 and 916, in which a shortcircuit is not caused; therefore, the current flowing to the memoryprovided in the opening 915 is dominant for a current value which flowsto the whole memory cell. Next, when the second writing is performed, ashort circuit is caused between the opposite electrode and the lowerelectrode at the opening 914. Therefore, the sum of the current flowingto the memory provided in the opening 915 and the current flowing to thememory provided in the opening 914 is dominant for a current whichsupplies to the whole memory cell. Similarly, a current which flows tothe whole memory cell after the third writing is the sum of the currentflowing to each of the memories provided in the opening 914, 915, and916, respectively. Since the contact resistance R1 to R3 of theconductive layers 911 to 913 and the opposite electrode 420 is relatedto R2>R1>R3, a ratio of the current value flowing after the firstwriting and the current value flowing after the second writing can beincreased and a margin for reading can be increased.

Further, description is made in detail by using the formulae. Aresistance value of the memory layer before a short circuit is denotedby Ra. A voltage applied to the memory element in reading is denoted byVr. A reading current value before reading is expressed by Formula 1shown in Embodiment Mode 2. Note that approximation is performed on theassumption that Ra>>R2>R1>R3 is satisfied. When the first writing isperformed, a short circuit is caused between the opposite electrode andthe lower electrode at the opening 915. A reading current value I1 afterthe short circuit is expressed by Formula 2 shown in Embodiment Mode 2.Note that approximation is performed on the assumption that Ra>>R2>R1>R3is satisfied. At this time, a ratio of the current values before andafter the first writing is expressed by Formula 3 shown in EmbodimentMode 2.

It can be said that the ratio of the reading current is sufficientlylarge since Ra>>R1 is satisfied. Next, when the second writing isperformed, a short circuit is caused between the opposite electrode andthe lower electrode at the opening 914. A reading current value I2 afterthe short circuit is expressed by Formula 4 shown in Embodiment Mode 2.Note that approximation is performed on the assumption that Ra>>R2>R1>R3is satisfied. At this time, a ratio of the current values before andafter the second writing is expressed by Formula 5 shown in EmbodimentMode 2. The ratio of the reading current can be increased by making R2sufficiently larger than R1. Next, when the third writing is performed,a short circuit is caused between the opposite electrode and the lowerelectrode at the opening 916. A reading current value I3 after the shortcircuit is expressed by Formula 6 shown in Embodiment Mode 2. Note thatapproximation is performed on the assumption that Ra>>R1>R2>R3 issatisfied. At this time, a ratio of the current values before and afterthe third writing is expressed by Formula 7 shown in Embodiment Mode 2.At this time, the ratio of the reading current can be increased bymaking R3 sufficiently larger than R1 and R2.

Note that in this embodiment mode, although a margin for reading isincreased by using difference of contact resistance, it can also beincreased by using a material which satisfies R4>R5>R6 when resistantvalues of electrode materials of the conductive layers 911, 912, and 913to be the lower electrodes are denoted by R4, R5, and R6, respectively;therefore, a margin for reading can be increased.

As described above, in the invention, a memory cell can be multivaluedin one memory cell. Thus, storage capacity of the memory device can beincreased.

FIG. 10 shows a mode where the antenna 430 for supplying power or thelike to the memory element 426 is provided. This embodiment mode shows amode where the antenna 430 is formed at the opening provided in thepartition wall.

The antenna 430 can be formed to be electrically connected to the thinfilm transistor provided in the memory element region 201 and is formedof a conductive material, preferably a low-resistance material such asCu (copper), Ag (silver), or Al (aluminum). Further, the antenna 430 ispreferably formed to be thick in order to lower the resistance thereof.The antenna 430 can be formed by a vapor deposition method, a printingmethod, a plating method, or a droplet discharging method typified by anink-jet method.

By forming the antenna 430 and the thin film transistor over the samesubstrate in this manner, wireless communication with a reader/writerdevice can be performed. As a result, multivalued information can beobtained from the memory element 426 without breakdown.

Note that this embodiment mode can be freely implemented in combinationwith Embodiment Mode 1 or 2.

Embodiment Mode 4

In this embodiment mode, a method of forming a memory element over aglass substrate as an insulating substrate is described. A method offorming the memory element over a plurality of electrode steps isdescribed. Note that a mode in which the memory element and a circuit (acontrol circuit) for controlling the memory element are formed over thesame substrate is shown. In addition, parts of the steps same asEmbodiment Mode 1 are described using the same drawings and the samereference numerals.

First, in the same manner as in FIG. 1A, the separation layer 402 isformed over the glass substrate 401. Quartz or the like is used for theinsulating substrate as well as glass. The separation layer 402 isobtained by forming a film containing metal or a film containing siliconentirely or selectively over the substrate.

Next, in the same manner as in Embodiment Mode 1, the insulating layer403 is formed so as to cover the separation layer 402. The insulatinglayer 403 is formed of silicon oxide, silicon nitride, or the like.Then, the semiconductor layer is formed over the insulating layer 403and crystallized by laser crystallization, thermal crystallization usinga metal catalyst, or the like, and then patterned into a desired shapeto form the island-shaped semiconductor layer. The laser crystallizationmay be performed by using a continuous wave laser or a pulsed laser.

Next, in the same manner as in Embodiment Mode 1, the gate insulatinglayer 405 is formed so as to cover the semiconductor layer. The gateinsulating layer 405 is formed using silicon oxide, silicon nitride, orthe like. The gate insulating layer 405 can be formed by a CVD method, athermal oxidation method, or the like. Alternatively, the semiconductorlayer and the gate insulating layer 405 may be continuously formed by aCVD method, and subsequently can be patterned at the same time. In thiscase, impurity contamination at the boundary between each layer can besuppressed.

Then, in the same manner as in Embodiment Mode 1, the gate electrodelayer 406 is formed. The gate electrode layer 406 is formed by using anelement selected from tantalum (Ta), tungsten (W), titanium (Ti),molybdenum (Mo), aluminum (Al), and copper (Cu), or an alloy material ora compound material including such elements as a main component, andpatterned into a desired shape. In the case of patterning by aphotolithography method, the width of the gate electrode can be madenarrower by using a resist mask of which width is narrowed by plasmaetching or the like. Accordingly, performance of a transistor can beimproved. The gate electrode layer 406 may have either a single-layerstructure or a stacked-layer structure.

Next, in the same manner as in Embodiment Mode 1, impurity elements areadded to the semiconductor layer to form the impurity region 407. Theimpurity region 407 is formed by using a resist mask formed by aphotolithography method and adding an impurity element such asphosphorus, arsenic, or boron. With the impurity element, a polarity ofeither an n-channel type or a p-channel type can be determined.

Then, in the same manner as in Embodiment Mode 1, as shown in FIG. 1B,the insulating layer is formed of an insulator containing silicon, forexample, silicon nitride, and the insulating layer is anisotropically(perpendicularly) etched to form the insulating layer (also referred toas the sidewall) 409 which contacts with a side surface of the gateelectrode. When the sidewall is formed, the gate insulating layer 405might be etched.

Next, in the same manner as in Embodiment Mode 1, impurities are furtheradded to the semiconductor layer to form the first impurity region 410right under the insulating layer (sidewall) 409 and the second impurityregion 411 having a higher concentration of impurities than the firstimpurity region 410.

Then, in the same manner as in Embodiment Mode 1, the insulating layeris formed so as to cover the semiconductor layer and the gate electrodelayer 406. The insulating layer is formed using an inorganic material,an organic material, or the like, which has an insulating property. Asan inorganic material having an insulating property, silicon oxide,silicon nitride, or the like can be used. As an organic material havingan insulating property, polyimide, acrylic, polyamide, polyimide amide,resist, benzocyclobutene, siloxane, or polysilazane can be used.

Here, as shown in FIG. 1C, a mode is shown where the insulating layer isformed to have a stacked-layer structure, and the first insulating layer414 a, the second insulating layer 414 b, and the third insulating layer414 c are formed in this order from the bottom. The first insulatinglayer 414 a is preferably formed by a plasma CVD method so as to containmuch hydrogen since dangling bonds of the semiconductor layer can bereduced by hydrogen. The second insulating layer 414 b is preferablyformed using an organic material since planarity can be improved. Thethird insulating layer 414 c is preferably formed using an inorganicmaterial in order to prevent discharge of moisture or the like from thesecond insulating layer 414 b formed of an organic material or toprevent intrusion of moisture through the second insulating layer 414 b.

Next, the contact hole is formed in the insulating layer to expose thesecond impurity region 411, and similarly to FIG. 1D, the conductivelayer 415 is formed so as to fill the contact hole. The conductive layer415 includes a film formed of an element selected from aluminum (Al),titanium (Ti), molybdenum (Mo), and tungsten (W), an alloy filmincluding such elements, an alloy film including such elements andsilicon, or the like. Further, the conductive layer 415 can be formed tohave a single-layer structure or a stacked-layer structure. After that,the conductive layer 415 is patterned into a desired shape, and a sourceelectrode, a drain electrode, and other electrodes are formed at thesame time.

In order to lower contact resistance between the source electrode andthe drain electrode; and the second impurity region 411, a silicide maybe formed on the impurity region. For example, a film including a metalelement (typically, Ni) is formed on the second impurity region 411 andheated by a thermal annealing method using an annealing furnace, a laserannealing method, or a rapid thermal annealing method (RTA method). As aresult, a silicide including the metal element and silicon is formed onthe second impurity region; therefore, improvement in on current ormobility can be realized.

In this manner, the thin film transistors are completed in the controlcircuit portion 202 and the memory element region 201. In the controlcircuit portion 202, a circuit is formed by using the thin filmtransistors.

Next, in the same manner as in Embodiment Mode 1, the insulating layer416 is formed so as to cover the conductive layer 415. The insulatinglayer 416 may be formed using an inorganic material, an organicmaterial, or the like which has an insulating property, and may have asingle-layer structure or a stacked-layer structure. An inorganicmaterial or an organic material similar to those of the first insulatinglayer 414 a, the second insulating layer 414 b, and the third insulatinglayer 414 c can be used.

After that, as shown in FIG. 11A, the insulating layer 416 is formed ina contact hole so as to expose the conductive layer 415, and theconductive layers 903 and 904 are stacked so as to fill the contacthole. Each of the conductive layers 903 and 904 includes a film formedof an element selected from aluminum (Al), titanium (Ti), molybdenum(Mo), and tungsten (W), an alloy film including such elements, an alloyfilm including such elements and silicon, or the like. Alternatively,the conductive layers 903 and 904 may be formed by using a lighttransmitting material such as indium tin oxide (ITO), indium tin oxidecontaining silicon oxide, or indium oxide containing 2 to 20% of zincoxide. In FIG. 11A, titanium is used for the conductive layer 903 andaluminum is used for the conductive layer 904.

Next, as shown in FIG. 11B, a taper angle of the conductive layer 903 ismade smaller than a taper angle of the conductive layer 904. Here, aside surface of the conductive layer 904 is inclined at approximately90° with respect to a substrate surface. Although the angle is nottapered, it is referred to as a taper angle. Further, the taper angle ofthe conductive layer 904 is approximately 45°. The conductive layers 903and 904 are processed so as to expose a surface of the conductive layer904, so that two steps having different taper angles can be provided. Itis considered that the lower a breakdown voltage of the memory elementis, the larger a taper angle is; therefore, by providing two stepshaving different taper angles using the lower electrode, two memorieshaving a different breakdown voltage of the memory element can beformed. That is, the conductive layers 903 and 904 can function as thelower electrodes of the memory element and also as the steps foradjusting the breakdown voltage of the memory element.

Next, as shown in FIG. 12A, an insulating layer is formed so as to coverthe conductive layers 903 and 904, and a plurality of openings areprovided. The openings 905, 906, and 907 of the partition wall 418 areformed by etching.

As described above, a plurality of openings and the conductive layerhaving a plurality of steps can be formed.

Note that although this embodiment mode shows the case where the lowerelectrode of the memory element is formed of the conductive layers 903and 904, the conductive layer 415 to be a source electrode or a drainelectrode of a thin film transistor may be used in common for the lowerelectrode of the memory element.

Next, as shown in FIG. 12B, the material layer 408 of the memory elementis formed in the opening of the partition wall. The material layer 408of the memory element can be formed by a vapor deposition method, a spincoating method, or a droplet discharging method typified by an ink-jetmethod.

Since the material layer 408 of the memory element can also be formedusing the same material as the electroluminescent layer included in thelight emitting element, the memory element and the light emittingelement can be formed over the same substrate. That is, a memory devicehaving a display function can be formed.

Next, the conductive layer to be the opposite electrode 420 is formed.Since the opposite electrode 420 can be formed over the whole surface ofthe memory element region, patterning by a photolithography method isnot required. Needless to say, the opposite electrode 420 may beselectively formed by patterning. The opposite electrode 420 canfunction as the upper electrode of the memory element.

Thus, the memory element 426 including the conductive layer 417, thematerial layer 408 of the memory element, and the opposite electrode 420is formed. In one memory cell, three memory elements corresponding tothe three openings 905, 906, and 907 are formed, and this memory cellhas three voltage values by which electric resistance is changed.

In this manner, in the invention, a memory cell can be multivalued inone memory cell. Thus, storage capacity of a memory device can beincreased.

In addition, according to Embodiment Mode 1, an antenna for supplyingpower or the like to the memory element 426 can be provided. The antennacan be formed to be connected to the thin film transistor provided inthe memory element region 201 and is formed of a conductive material,preferably a low-resistance material such as Cu (copper), Ag (silver),or Al (aluminum).

Although a semiconductor device including the memory element region andthe antenna can be completed through the aforementioned steps, the glasssubstrate 401 may be peeled off later in the steps shown in EmbodimentMode 1.

Then, a flexible resin substrate is attached instead of the peeled glasssubstrate 401.

As a result of peeling off the glass substrate 401 in this manner,reduction in thickness and weight of a semiconductor device includingthe memory element can be achieved and flexibility and impact resistancethereof can be improved.

Note that this embodiment mode can be freely implemented in combinationwith Embodiment Mode 1, 2, or 3.

Embodiment Mode 5

In this embodiment mode, a method of forming a memory element over aglass substrate as an insulating substrate is described. The memoryelement is formed over a plurality of electrode steps. Note that a modein which the memory element and a circuit (a control circuit) forcontrolling the memory element are formed over the same substrate isshown. In addition, parts of the steps same as those in Embodiment Mode1 are described using the same drawings and the same reference numerals.

First, in the same manner as in FIG. 1A, the separation layer 402 isformed over the glass substrate 401. Quartz or the like is used for theinsulating substrate as well as glass. The separation layer 402 isobtained by forming a film containing metal or a film containing siliconentirely or selectively over the substrate.

Next, in the same manner as in Embodiment Mode 1, the insulating layer403 is formed so as to cover the separation layer 402. The insulatinglayer 403 is formed of silicon oxide, silicon nitride, or the like.Then, the semiconductor layer is formed over the insulating layer 403and crystallized by laser crystallization, thermal crystallization usinga metal catalyst, or the like, and then patterned into a desired shapeto form the island-shaped semiconductor layer. The laser crystallizationmay be performed by using a continuous wave laser or a pulsed laser.

Next, in the same manner as in Embodiment Mode 1, the gate insulatinglayer 405 is formed so as to cover the semiconductor layer. The gateinsulating layer 405 is formed using silicon oxide, silicon nitride, orthe like. The gate insulating layer 405 can be formed by a CVD method, athermal oxidation method, or the like. Alternatively, the semiconductorlayer and the gate insulating layer 405 may be continuously formed by aCVD method, and subsequently can be patterned at the same time. In thiscase, impurity contamination at the boundary between each layer can besuppressed.

Then, in the same manner as in Embodiment Mode 1, the gate electrodelayer 406 is formed. The gate electrode layer 406 is formed by using anelement selected from tantalum (Ta), tungsten (W), titanium (Ti),molybdenum (Mo), aluminum (Al), and copper (Cu), or an alloy material ora compound material including such elements as a main component, andpatterned into a desired shape. In the case of patterning by aphotolithography method, the width of the gate electrode can be madenarrower by using a resist mask of which width is narrowed by plasmaetching or the like. Accordingly, performance of a transistor can beimproved. The gate electrode layer 406 may have either a single-layerstructure or a stacked-layer structure.

Next, in the same manner as in Embodiment Mode 1, impurity elements areadded to the semiconductor layer to form the impurity region 407. Theimpurity region 407 is formed by using a resist mask formed by aphotolithography method and adding an impurity element such asphosphorus, arsenic, or boron. With the impurity element, a polarity ofeither an n-channel type or a p-channel type can be determined.

Then, in the same manner as in Embodiment Mode 1, as shown in FIG. 1B,the insulating layer is formed of an insulator containing silicon, forexample, silicon nitride, and the insulating layer is anisotropically(perpendicularly) etched to form the insulating layer (also referred toas the sidewall) 409 which contacts with a side surface of the gateelectrode. When the sidewall is formed, the gate insulating layer 405might be etched.

Next, in the same manner as in Embodiment Mode 1, impurities are furtheradded to the semiconductor layer to form the first impurity region 410right under the insulating layer (sidewall) 409 and the second impurityregion 411 having a higher concentration of impurities than the firstimpurity region 410.

Then, in the same manner as in Embodiment Mode 1, the insulating layeris formed so as to cover the semiconductor layer and the gate electrodelayer 406. The insulating layer is formed using an inorganic material,an organic material, or the like which has an insulating property. As aninorganic material having an insulating property, silicon oxide, siliconnitride, or the like can be used. As an organic material having aninsulating property, polyimide, acrylic, polyamide, polyimide amide,resist, benzocyclobutene, siloxane, or polysilazane can be used.

Here, as shown in FIG. 1C, a mode is shown where the insulating layer isformed to have a stacked-layer structure, and the first insulating layer414 a, the second insulating layer 414 b, and the third insulating layer414 c are formed in this order from the bottom. The first insulatinglayer 414 a is preferably formed by a plasma CVD method so as to containmuch hydrogen since dangling bonds of the semiconductor layer can bereduced by hydrogen. The second insulating layer 414 b is preferablyformed using an organic material since planarity can be improved. Thethird insulating layer 414 c is preferably formed using an inorganicmaterial in order to prevent discharge of moisture or the like from thesecond insulating layer 414 b formed of an organic material or toprevent intrusion of moisture through the second insulating layer 414 b.

Next, the contact hole is formed in the insulating layer to expose thesecond impurity region 411, and as shown in FIG. 1D, the conductivelayer 415 is formed so as to fill the contact hole. The conductive layer415 includes a film formed of an element selected from aluminum (Al),titanium (Ti), molybdenum (Mo), and tungsten (W), an alloy filmincluding such elements, an alloy film including such elements andsilicon, or the like. Further, the conductive layer 415 can be formed tohave a single-layer structure or a stacked-layer structure. After that,the conductive layer 415 is patterned into a desired shape, and a sourceelectrode, a drain electrode, and other electrodes are formed at thesame time.

In order to lower contact resistance between the source electrode andthe drain electrode; and the second impurity region 411, a silicide maybe formed on the impurity region. For example, a film including a metalelement (typically, Ni) is formed on the second impurity region 411 andheated by a thermal annealing method using an annealing furnace, a laserannealing method, or a rapid thermal annealing method (RTA method). As aresult, a silicide including the metal element and silicon is formed onthe second impurity region; therefore, improvement in on current ormobility can be realized.

In this manner, the thin film transistors are completed in the controlcircuit portion 202 and the memory element region 201. In the controlcircuit portion 202, a circuit is formed by using the thin filmtransistors.

Next, in the same manner as in Embodiment Mode 1, the insulating layer416 is formed so as to cover the conductive layer 415. The insulatinglayer 416 may be formed using an inorganic material, an organicmaterial, or the like which has an insulating property, and may have asingle-layer structure or a stacked-layer structure. An inorganicmaterial or an organic material similar to those of the first insulatinglayer 414 a, the second insulating layer 414 b, and the third insulatinglayer 414 c can be used.

After that, as shown in FIG. 14A, a contact hole is formed in theinsulating layer 416 so as to expose the conductive layer 415, and theconductive layer 903 is formed so as to fill the contact hole. Theconductive layer 903 includes a film formed of an element selected fromaluminum (Al), titanium (Ti), molybdenum (Mo), and tungsten (W), analloy film including such elements, an alloy film including suchelements and silicon, or the like. Alternatively, the conductive layer903 may be formed by using a light transmitting material such as indiumtin oxide (ITO), indium tin oxide containing silicon oxide, or indiumoxide containing 2 to 20% of zinc oxide. In FIG. 14A, titanium is usedfor the conductive layer 903.

Next, the conductive layer 903 is processed into a desired shape. It isconsidered that the higher a step is, the lower a breakdown voltage ofthe memory element is; therefore, by providing two steps each havingdifferent height using the lower electrode, two memories having adifferent breakdown voltage of the memory element can be formed. First,as shown in FIG. 14B, the conductive layer 903 is processed, and then, apart of the conductive layer 903 is further processed as shown in FIG.14C. Technique such as half etching is used for processing theconductive layer 903. In this manner, the conductive layer 903 can beprovided with two steps having different height. The conductive layer903 can function as the lower electrode of the memory element and alsoas the step for adjusting the breakdown voltage of the memory element.Further, when a light exposure method using a light exposure maskincluding a semi-transmission portion, which is referred to as ahalf-tone light exposure method, is used, the conductive layer 903 canbe processed in a short time. Alternatively, a photo mask or a reticleeach of which is provided with an auxiliary pattern having a lightintensity reducing function, which is formed of a diffraction gratingpattern, may be applied to a photolithography step for forming theconductive layer 903.

Next, as shown in FIG. 15A, an insulating layer is formed so as to coverthe conductive layer 903, and a plurality of openings are provided. Thepartition wall 418 provided with the openings 905, 906, and 907 isformed.

As described above, a plurality of openings and the conductive layerhaving a plurality of steps can be formed.

Note that although this embodiment mode shows the case where the lowerelectrode of the memory element is formed of the conductive layer 903,the conductive layer 415 to be a source electrode or a drain electrodeof a thin film transistor may be used in common for the lower electrodeof the memory element.

Next, as shown in FIG. 15B, the material layer 408 of the memory elementis formed in the opening of the partition wall. The material layer 408of the memory element can be formed by a vapor deposition method, a spincoating method, or a droplet discharging method typified by an ink-jetmethod.

Since the material layer 408 of the memory element can also be formedusing the same material as the electroluminescent layer included in thelight emitting element, the memory element and the light emittingelement can be formed over the same substrate through a common step.That is, a memory device having a display function can be formed.

Next, the conductive layer to be the opposite electrode 420 is formed.Since the opposite electrode 420 can be formed over the whole surface ofthe memory element region, patterning by a photolithography method isnot required. Needless to say, the opposite electrode 420 may beselectively formed by patterning. The opposite electrode 420 canfunction as the upper electrode of the memory element.

Thus, the memory element 426 including the conductive layer 417, thematerial layer 408 of the memory element, and the opposite electrode 420is formed. In one memory cell, three memory elements corresponding tothe three openings 905, 906, and 907 are formed, and this memory cellhas three voltage values by which electric resistance is changed.

In this manner, in the invention, a memory cell can be multivalued inone memory cell. Thus, storage capacity of a memory device can beincreased.

In addition, according to Embodiment Mode 1, an antenna for supplyingpower or the like to the memory element 426 can be provided. The antennacan be formed to be connected to the thin film transistor provided inthe memory element region 201 and is formed of a conductive material,preferably a low-resistance material such as Cu (copper), Ag (silver),or Al (aluminum).

Although a semiconductor device including the memory element region andthe antenna can be completed through the aforementioned steps, the glasssubstrate 401 may be peeled off later in the steps shown in EmbodimentMode 1.

Then, a flexible resin substrate may be attached instead of the peeledglass substrate 401.

As a result of peeling off the glass substrate 401 in this manner,reduction in thickness and weight of a semiconductor device includingthe memory element can be achieved and flexibility and impact resistancethereof can be improved.

Note that this embodiment mode can be freely implemented in combinationwith Embodiment Mode 1, 2, 3, or 4.

Embodiment Mode 6

A structure of a semiconductor device in this embodiment mode isdescribed with reference to FIG. 16A. As shown in FIG. 16A, asemiconductor device 620 of the invention has a function ofcommunicating data without contact, and includes a power supply circuit611, a clock generation circuit 612, a data demodulation/modulationcircuit 613, a control circuit 614 for controlling other circuits, aninterface circuit 615, a storage circuit 616 including a plurality ofmemory cells which can store multivalued information, a data bus 617, anantenna (antenna coil) 618, a sensor 621, and a sensor circuit 622.

The power supply circuit 611 generates various kinds of power supplies,which are supplied to each circuit in the semiconductor device 620, inaccordance with an alternating current signal input from the antenna618. The clock generation circuit 612 generates various kinds of clocksignals, which are supplied to each circuit in the semiconductor device620, in accordance with an alternating current signal input from theantenna 618. The data demodulation/modulation circuit 613 has a functionof demodulating/modulating data communicated with a reader/writer 619.The control circuit 614 has a function of controlling the memory circuit616 including a plurality of memory cells which can store multivaluedinformation. The antenna 618 has a function of transmitting/receivingelectromagnetic fields or electric waves. The reader/writer 619communicates with and controls the semiconductor device, and controls aprocess with regard to the data of the semiconductor device. Note thatthe structure of the semiconductor device is not limited to the abovestructure, and for example, other elements such as a limiter circuit ofa power supply voltage and hardware dedicated to encryption may beadditionally provided.

The memory circuit 616 including a plurality of memory cells which canstore multivalued information has a memory element where an insulatinglayer which is changed by an external electric action is interposedbetween a pair of conductive layers. Note that the memory circuit 616including a plurality of memory cells which can store multivaluedinformation may have only the memory element where an insulating layeris interposed between a pair of conductive layers, or may have a memorycircuit with a different structure. The memory circuit with a differentstructure corresponds to one or more selected from a DRAM, an SRAM, amask ROM, a PROM, an EPROM, an EEPROM, and a flash memory, for example.

A sensor 621 is formed using a semiconductor element such as a resistorelement, a capacitive coupling element, an inductive coupling element, aphotovoltaic element, a photoelectric conversion element, athermoelectric element, a transistor, a thermistor, or a diode. Thesensor circuit 622 detects changes in impedance, reactance, inductance,voltage, or current, and performs analog/digital conversion (A/Dconversion) to output a signal to the control circuit 614.

Next, one mode of an electronic apparatus on which the semiconductordevice of the invention is mounted is described with reference todrawings. The electronic apparatus shown here is a portable phoneincluding chassis 700 and 706, a panel 701, a housing 702, a printedwiring board 703, an operation switch 704, and a battery 705 (see FIG.16B). The panel 701 is detachably incorporated in the housing 702. Thehousing 702 is fitted into the printed wiring board 703. A shape anddimension of the housing 702 are changed as appropriate in accordancewith the electronic apparatus in which the panel 701 is incorporated. Onthe printed wiring board 703, a plurality of packaged semiconductordevices are mounted, and the semiconductor device of the invention canbe used as one of them. A plurality of semiconductor devices mounted onthe printed wiring board 703 has any one of functions of a controller, acentral processing unit (CPU), a memory, a power supply circuit, anaudio processing circuit, a transmitting/receiving circuit, and thelike.

The panel 701 is fixedly connected with the printed wiring board 703through a connection film 708. The panel 701, the housing 702, and theprinted wiring board 703 described above are placed in the chassis 700and 706 together with the operation switch 704 and the battery 705. Apixel region 709 included in the panel 701 is provided so as to beobserved through an opening window provided in the chassis 700.

As described above, the semiconductor device of the invention is small,thin, and lightweight, and thereby the limited space in the chassis 700and 706 of the electronic apparatus can be used efficiently.

In addition, since the semiconductor device of the invention uses amemory element having a simple structure in which an insulating layerwhich is changed by an external electric action (namely, a layerincluding an organic compound interposed between a pair of electrodes),as a memory included in the semiconductor device, is interposed betweena pair of conductive layers, an electronic apparatus using aninexpensive semiconductor device can be provided. Further, since thesemiconductor device of the invention includes a plurality of memorycells which can store multivalued information and high integration ofthe semiconductor device is easy, an electronic apparatus using asemiconductor device including a storage circuit of which capacity perunit area is large can be provided.

Note that the chassis 700 and 706 are shown as an example of an exteriorof the mobile phone, and the electronic apparatus according to thisembodiment mode can be changed variously in accordance with the functionor the intended purpose thereof.

In addition, another mode of an electronic device including asemiconductor device of the invention is described with reference toFIG. 17A. The electronic apparatus shown here is a mobile musicreproduction device equipped with a recording medium, which includes amain body 2901, a display portion 2903, a recording medium 2907 (a cardtype memory, a compact large capacity memory, or the like) readingportion, operation keys 2902 and 2906, a speaker portion 2905 of aheadphone connected to a connection cord 2904, and the like. Since thesemiconductor device of the invention includes a plurality of memorycells which can store multivalued information and high integration ofthe semiconductor device is easy, a storage circuit of which capacityper unit area is large can be applied to the recording medium 2907 and alightweight music reproduction device can be achieved. Further, since amemory and an antenna can be formed over the same substrate, the antennais integrated in the recording medium 2907 and thus the musicreproduction device can be made smaller. By integrating the antenna, themobile music reproduction device can perform wireless communication witha reader/writer device.

In addition, another mode of an electronic apparatus including asemiconductor device of the invention is described with reference toFIG. 17B. The electronic apparatus shown here is a portable computerwhich can be worn around an arm, which includes a main body 2911, adisplay portion 2912, a switch 2913, an operation key 2914, a speakerportion 2915, a semiconductor integrated circuit 2916, and the like.Various input or operation can be performed by the display portion 2902which serves as a touch panel. Further, although not shown here, thisportable computer is provided with a cooling function for suppressingthe increase of its temperature, an infrared port, and a communicationfunction such as a high-frequency circuit.

A portion which touches a human arm is preferably covered with a filmsuch as plastic so that he/she does not feel uncomfortable even when ittouches a human arm 2910. Accordingly, it is preferable to form thesemiconductor integrated circuit 2916 (a memory, a CPU, or the like) andthe display portion 2912 over a plastic substrate. Further, an externalshape of the main body 2911 may be curved along the human arm 2910. Theinvention can store multivalued information, and a storage circuit ofwhich capacity per unit area is large is formed over a flexible resinsubstrate so as to be used as a part of the semiconductor integratedcircuit 2916, so that a flexible portable computer can be achieved.

Further, a storage circuit of the invention is applied to thesemiconductor integrated circuit 2916 (a memory, a CPU, a high-frequencycircuit, or the like), which is included in the portable computer, acontrol circuit of the speaker portion 2915, and the like; therefore,mounted components of the portable computer can be reduced. For example,by forming a memory and an antenna over the same substrate as shown inEmbodiment Mode 1, the portable computer can perform wirelesscommunication with a reader/writer device. Since manufacturing cost canbe reduced by using a storage circuit including a plurality of memorycells which can store multivalued information, of which capacity perunit area is large, a portable computer can be provided at low cost.

Note that this embodiment mode can be freely implemented in combinationwith Embodiment Mode 1, 2, 3, 4, or 5.

Embodiment Mode 7

According to the invention, a semiconductor device including a pluralityof memory cells which can store multivalued information and functioningas a radio chip can be formed. The radio chip can be applied to varioususes such as bills, coins, securities, bearer bonds, documents (such asdriver's licenses or resident's cards, see FIG. 18A), packagingcontainers (such as wrapping paper or bottles, see FIG. 18C), storagemedia (such as DVD software or video tapes, see FIG. 18B), vehicles(such as bicycles, see FIG. 18D), personal belongings (such as bags orglasses), foods, plants, animals, human bodies, clothing, everydayarticles, tags on goods such as electronic apparatus or on packs (seeFIGS. 18E and 18F). Note that an electronic apparatus includes a liquidcrystal display device, an EL display device, a television set (alsosimply called as a TV set, a TV receiver, or a television receiver), amobile phone, and the like.

A semiconductor device 910 of the invention is fixed to a product bybeing mounted on a printed substrate, attached to a surface of theproduct, embedded inside the product, or the like. For example, if theproduct is a book, the semiconductor device 910 is embedded in paper,and if the product is a package made from an organic resin, thesemiconductor device 910 is embedded in the organic resin. Since thesemiconductor device 910 of the invention can have a small size, a thinshape, and light weight, the design quality of the product itself is notdegraded even after the semiconductor device is fixed to the product. Byproviding the semiconductor device 910 in bills, coins, securities,bearer bonds, documents, and the like, a certification function can beprovided and forgery can be prevented through use of the certificationfunction. Further, when the semiconductor device of the invention isprovided in packaging containers, storage media, personal belongings,foods, plants, clothing, everyday articles, electronic apparatuses, andthe like, systems such as an inspection system can be more efficient.

Note that this embodiment mode can be freely implemented in combinationwith Embodiment Mode 1, 2, 3, 4, 5, or 6.

In the invention, an electrode of a memory element is preciselyprocessed; therefore, variation of a writing voltage or a readingvoltage of a plurality of memory cells can be reduced, and high yieldcan be achieved in a mass production line.

This application is based on Japanese Patent Application serial No.2006-047057 filed in Japan Patent Office on Feb. 23, 2006, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device including at least onememory cell, the memory cell comprising: a first storage element, asecond storage element, and a third storage element, a first electrode,a second electrode, and a material layer between the first electrode andthe second electrode, wherein the first electrode, the second electrode,and the material layer between the first electrode and the secondelectrode are included in the first storage element, the second storageelement, and the third storage element, wherein the first electrode hasa stacked-layer structure including a bottom layer and a top layer, andwherein a taper angle of a side surface of the top layer is differentfrom a taper angle of a side surface of the bottom layer.
 2. Thesemiconductor device according to claim 1, wherein storage capacitiesper unit area of the first storage element, the second storage element,and the third storage element are different from one another.
 3. Thesemiconductor device according to claim 1, wherein, in each of thefirst, second, third storage elements, an electrical resistance changesdue to an application of a voltage thereto, and wherein a voltage valueat which the voltage changes in the first storage element is differentfrom those in the second and third storage elements.
 4. A semiconductordevice including at least one memory cell, the memory cell comprising: afirst storage element, a second storage element, and a third storageelement, a first electrode, a second electrode, and a material layerbetween the first electrode and the second electrode, wherein the firstelectrode, the second electrode, and the material layer between thefirst electrode and the second electrode are included in the firststorage element, the second storage element, and the third storageelement, wherein the first electrode has a stacked-layer structureincluding a bottom layer and a top layer, wherein an end portion of thebottom layer extends beyond an end portion of the top layer, wherein inthe first storage element, an end portion of the bottom layer, thematerial layer, and the second electrode are overlapped, wherein in thesecond storage element, the bottom layer, an end portion of the toplayer, the material layer, and the second electrode are overlapped, andwherein in the third storage element, the bottom layer, the top layer,the material layer, and the second electrode are overlapped.
 5. Thesemiconductor device according to claim 4, wherein storage capacitiesper unit area of the first storage element, the second storage element,and the third storage element are different from one another.
 6. Thesemiconductor device according to claim 4, wherein taper angle of a sidesurface of the top layer is different from that of the bottom layer. 7.The semiconductor device according to claim 4, wherein, in each of thefirst, second, and third storage elements, an electrical resistancechanges due to an application of a voltage thereto, and wherein avoltage value at which the voltage changes in the first storage elementis different from those in the second and third storage elements.
 8. Asemiconductor device comprising: a first storage element and a secondstorage element, a first electrode, a second electrode, and a materiallayer between the first electrode and the second electrode, wherein thefirst electrode, the second electrode, and the material layer betweenthe first electrode and the second electrode are included in the firststorage element and the second storage element, wherein the firstelectrode has a stacked-layer structure including a bottom layer and atop layer, and wherein a taper angle of a side surface of the top layeris different from a taper angle of a side surface of the bottom layer.9. The semiconductor device according to claim 8, wherein storagecapacities per unit area of the first storage element and the secondstorage element are different from each other.
 10. The semiconductordevice according to claim 8, wherein storage capacities per unit area ofthe first storage element and the second storage element are differentfrom each other.
 11. A semiconductor device comprising: a first storageelement and a second storage element, a first electrode, a secondelectrode, and a material layer between the first electrode and thesecond electrode, wherein the first electrode, the second electrode, andthe material layer between the first electrode and the second electrodeare included in the first storage element and the second storageelement, wherein in the first storage element, an end portion of thefirst electrode, the material layer, and the second electrode areoverlapped, and wherein in the second storage element, the firstelectrode, the material layer, and the second electrode are overlapped.12. The semiconductor device according to claim 11, wherein, in each ofthe first storage element and the second storage element, an electricalresistance changes due to an application of a voltage thereto, andwherein a voltage value at which the voltage changes in the firststorage element is different from that in the second storage element.13. The semiconductor device according to claim 11, wherein, in each ofthe first storage element and the second storage element, an electricalresistance changes due to an application of a voltage thereto, andwherein a voltage value at which the voltage changes in the firststorage element is different from that in the second storage element.